Isn't this, like, the perfect deal? They could tell the origin of the characters in X-Men: DOFP and have any Magneto-related storylines there, but have Pietro and Wanda as Avengers with superpowers and not dwell on their origin at all, or play with words to reference it, Joss can absolutely do that. Moreover, Peters could be cast as a 'past' Quicksilver, on par with Fassbender's Magneto, but in the Avengers Pietro could be played by an older actor.